In a semiconductor element formed by using a semiconductor wafer, macro defects (shape defects) (irregularities, triangular defects, micro pipes, carrots, comets, downfalls and step bunching) and crystal defects (threading screw dislocation, threading edge dislocation and stacking faults) of a semiconductor wafer give serious influences to the performance, yield and reliability of the element. In particular, a silicon carbide wafer for use in a power controlling semiconductor element contains macro defects and crystal defects, and it is very important to inspect the defects of a wafer prior to the fabrication of a semiconductor element. Therefore, the inspection should be a non-destructive inspection and needs to be carried out under the condition that gives no influence to the fabrication of the element.
It is difficult to form a silicon carbide wafer by a fusing method unlike a silicon wafer, and the silicon carbide wafer is formed by the crystal growth using a sublimation method and a chemical vapor deposition method. Consequently, it is very difficult to eliminate crystal defects in the current technique, and threading dislocations (threading edge dislocations and threading screw dislocations) having a density of 103 cm−2 to 104 cm−2 and basal plane defects (basal plane dislocations and stacking faults) having a dislocation density of 1 cm−2 or less are present. Moreover, in addition to the crystal defects, macro defects to which the surface shape is reflected are present at a dislocation density of 1 cm−2 or less. The macro defects can be inspected and measured by a method using an optical microscope. Furthermore, a technique for suppressing the occurrence of macro defects by utilizing a wafer planarization technique and an epitaxial growth technique has been developed.
The semiconductor element for use in power control is fabricated in the regions divided into dies, each having an area of about 1 mm×1 mm to about 5 mm×5 mm in a wafer plane. Therefore, there are some dies containing the above-mentioned basal plane defects and macro defects and other dies containing no such defects. On the other hand, each die contains 102 to 103 pieces of the above-mentioned threading dislocations.
As a material for the above-mentioned wafer, a silicon carbide wafer and a gallium nitride wafer are used in most cases. As the silicon carbide wafer, a silicon carbide wafer or a wafer having a silicon carbide epitaxial film formed on a silicon carbide wafer is used in most cases. Moreover, as the gallium nitride wafer, a wafer having a film formed by epitaxial growth on a silicon wafer, a sapphire wafer or a silicon nitride wafer is used in most cases. Furthermore, also in the silicon carbide wafer or the gallium nitride wafer, it is important to inspect the above-mentioned macro defects and crystal defects. The background art relating to the defect inspection for the silicon carbide wafer will be described below. The same is true for the gallium nitride wafer, unless otherwise specified.
For example, as the methods for inspecting macro defects, optical inspection methods such as those using a differential interference microscope or a laser scattering system have been known. This method can inspect even crystal defects as long as there is a characteristic on the surface shape (see Patent Document 1). Moreover, as the methods for inspecting crystal defects, an X-ray topography (see Patent Document 2), a transmission electron microscope method and an etch pit method have been known. However, the transmission electron microscope method and the etch pit method are inspection methods in which the wafer needs to be destroyed, and are not used for non-destructive inspection. Furthermore, in the case of the detection method using light, the resolution is subjected to the restriction of a wavelength limit of light.